Microstructural Evolution Of Rapidly Solidified TiAl Based Alloys

Rapidly solidified Ti-46Al-2Cr-2Nb-xY (x=0,0.5,1.0,1.5,2.0) and Ti-46Al-2Cr- xNb-0.3Y (x=2.0,4.0,6.0) (at.%) alloy ribbons were produced by melt-spinning method. The effects of Y and Nb additions, and cooling rate on the microstructure and mechanical properties of the rapidly solidified TiAl based alloys were investigated. The evolution of microstructure and mechanical properties of the rapidly solidified TiAl based alloys during annealing treatment was also investigated. The consolidation of the rapidly solidified TiAl based alloy ribbons was carried out using Spark Plasma Sintering, and the microstructure and mechanical properties of the sintered compacts were studied.The investigation indicated that Y addition greatly influences the phase constitutions and microstructures of rapidly solidified Ti-46Al-2Cr-2Nb-xY alloys. The microstructure of Y added alloys was significantly refined. The grain size decreased from 6μm to less than 1μm with increasing Y content from 0 to 2.0at.%. The volume fraction ofγphase and the ratio ofγ/α2 increased with increasing Y addition. The primary solidification phase changed fromβphase toαphase when Y content was higher than 1.5at.%. The microstructure of rapidly solidified TiAl based alloys has intimate relationship with Y content. The microstructure of the alloys with Y addition less than 1.0at.% was composed of equiaxedα2 phase and lamellar or massiveγphase at grain boundaries. The alloys with Y content higher than 1.5at.% present equiaxedβ/B2 phase and equiaxed or lamellarα2 andγphase locating at grain boundaries. Rapid solidification increases the solid solubility of Y atoms in TiAl based alloys, but no more than 0.5at.%. The unsolvable Y atoms forming Al2Y phase with grain size of about 40-100nm distributed uniformly in matrix. The hardness of rapidly solidified TiAl based alloys was related with the volume fraction ofα2 phase. The volume fraction ofα2 phase was decreased with Y additions, thus the hardness was reduced.Nb content has minor effect on phase constitutions and microstructure of rapidly solidified Ti-46Al-2Cr-xNb-0.3Y alloys. The primary solidification phase of Ti-46Al-2Cr-xNb-0.3Y alloys varied fromβphase in conventionally cast condition toαphase in rapid solidification process. The alloys with various Nb contents consist ofγandα2 phase, and the content ofα2 phase decreased with minor Nb additions (4at.%) and increased as Nb content was further increased. The microstructure of the rapidly solidified TiAl based alloys with various Nb contents was composed of equiaxed grains and lamellar structure. Nb additions promoted the formation of lamellar structure and a large amount of stacking faults and dislocations, but did not change the thickness of the lamella.Nnear-Newtonian cooling condition was adopted to estimate the cooling rate of melt-spun TiAl based alloys, to be 105-106K/s. The Ti-46Al-2Cr-4Nb-0.3Y alloy was selected as an example to investigate the effect of cooling rate on microstructure. The variation of cooling rate did not influence significantly the microstructure of Ti-46Al-2Cr-4Nb-0.3Y alloy, which was composed of equiaxed grains and lamellar structure. The lamellar grain sizes were reduced with increasing cooling rate. The alloys with lower cooling rate consisted ofγandβ/B2 phase. With increasing cooling rate, the amount ofβ/B2 phase decreased and the amount ofα2 phase increased. The phase constitutions in alloys with higher cooling rate (9.9×105K/s and 1.3×106K/s) wereγphase andα2 phase. Higher cooling rate leads to an increase of hardness and a decrease of bending ductility of Ti-46Al-2Cr-4Nb-0.3Y alloy. It is confirmed that the bend ductility relates with lattice distortion ofγphase. This relationship can be applied to forecasting the deforming properties ofγ-TiAl alloy by calculating the lattice distortion ofγphase.The variation of Nb content did not alter the microstructure evolution of the rapidly solidified Ti-46Al-2Cr-xNb-0.3Y alloys during annealing treatment. When annealed at 600-800oC, the metastableα2 phase transformed toγphase. At higher annealing temperature (840oC),αphase precipitated fromγphase in Ti-46Al-2Cr-4Nb-0.3Y alloy, ordering transformation ofγphase and continuous coarsening occurred in the alloys annealed at 920oC and 970oC, respectively. The lamellar structure was globularized during annealing at 1050oC. The dislocations resulted from rapid solidification arrayed in a line during annealing treatment, acting as sub-boundaries which refines the microstructure. The lattice distortion ofγphase and hardness increased with increasing annealing temperature.The melt-spun TiAl based ribbons were pulverized into fine powder particles by ball milling and subject to spark plasma sintering to consolidate into compacts. The powder particle size decreased with increasing ball milling time, and powders with an average particle size of less than 75μm was obtained after milling for 2h. Sintering the milled powders at 1200oC produced fully dense compact. Higher sintering temperature did not improve the densification evidently, however, the microstructure and phase constitution varied with sintering temperature. The dominant phases wereγandα2 phase in the bulk alloys sintered at 1200oC. With higher sintering temperature, the amount ofα2 phase decreased and the microstructure changed from equiaxed nearγgrain to near lamellar structure, together with a slight coarsening.The bulk alloy sintered at 1260oC with refined and homogeneous near lamellar structure was measured to reveal the best overall mechanical properties. The compressional fracture stress and compression ratio were 2984 MPa and 41.5%, respectively, at room temperature. The tensile fracture stress and ductility were 527.5 MPa and 5.9%, respectively, at 800oC. The strength and ductility of the sintered bulk alloys are higher than those of their cast counterpart.